The present invention relates to a copier, facsimile apparatus, printer or similar image forming apparatus and, more particularly, to an image forming apparatus capable of collecting a toner remaining on an image carrier after image transfer and conveying the collected toner to a developing unit to reuse it.
It is a common practice with the above-described type of image forming apparatus to electrostatically form a latent image on an image carrier, develop the latent image by a toner to produce a corresponding toner image, transfer the toner image to a paper sheet or similar transfer medium, and then fix the toner image on the medium. In this kind of image forming process, while a majority of the toner deposited on the latent image is transferred to and consumed by the transfer medium, the other toner is left untranferred on the image carrier and collected by cleaning means. The manner in which the toner is to remain on the image carrier is susceptible to ambient conditions. For example, in a hot and humid environment, the image transfer efficiency is lowered to cause a greater amount of toner to remain on the image carrier. A small amount of toner is also deposited in the areas of the image carrier other than the forming area and collected by the cleaning means. Further, a toner image representative of a reference density pattern which implements toner concentration control or similar process control is also left untransferred on the image carrier and collected by the cleaning means.
To use the toner collected by the cleaning means effectively, there has been proposed an image forming apparatus of the type conveying the collected toner from the cleaning means to developing means, as disclosed in Japanese Utility Model Laid-Open Publication No. 166264/1984 by way of example (referred to as Prior Art 1 hereinafter). Specifically, the collected toner is transported to and supplemented from above an unused or fresh toner existing in the developing section of the developing means or is transported to a toner storing section included in the developing means and mixed with a fresh toner there. The position where the developing means receives the collected toner depends on the type of the apparatus. For example, the apparatus may receive the collected toner in the vicinity of a developing roller present in the developing section or receive it in the toner storing section. It has also been proposed to omit the cleaning means and assign the cleaning function also to the developing means.
However, the problem with the collected toner is that the physical property thereof changes before reuse, compared to a fresh toner. For example, the collected toner contains paper dust and other impurities and has the particle size thereof reduced. Moreover, the collected toner has the frictional chargeability thereof lowered by the repetitive transfer charge and has the fluidity or pulverulent characteristic thereof degraded. In this condition, when the collected toner and fresh toner are mixed in the developing means, it is difficult to maintain the toner concentration uniform and the toner charge stable. In light of the above, Japanese Patent Laid-Open Publication No. 41079/1985 (referred to as Prior Art 2 hereinafter) teaches an image forming apparatus having a collected toner storing section above the developing section of the developing means in addition to a fresh toner storing section. The supplement of the collected toner from the collected toner storing section and the supplement of the fresh toner from the fresh toner storing section are adequately controlled.
Another problem with the collected toner is that it deteriorates due to the repetitive image forming process, i.e., it becomes difficult to charge and contains an inversely charged toner. Hence, when the collected toner is supplemented to the developing section in a great amount at a time, it is apt to contaminate the background of an image, fly out of the developing means. To eliminate this problem, Japanese Patent Laid-Open Publication No. 57365/1985 (referred to as Prior Art 3 hereinafter) proposes an image forming apparatus including an optical sensor responsive to the density of an image having been transferred. When the image density is lowered as determined by the sensor, the collected toner is continuously supplemented and the fresh toner is supplemented. When the image density is not lowered and the transfer ratio of a toner image to a sheet is lowered, a predetermined amount of fresh toner is supplemented or the recycling operation is interrupted. This prevents the collected toner from being supplemented alone when the image density is not lowered and, therefore, insures desirable image quality and satisfactory cleaning.
To transport the toner removed from the image carrier by the cleaning means to the developing means, use may be made of a screw conveyor, as disclosed in Japanese Patent Utility Model Laid-Open Publication No. 155044/1977 (referred to as Prior Art 4 hereinafter). Alternatively, the transport may be implemented by a screw for conveying the collected toner, an elastic rotatable plate disposed in a compartment, and a belt conveyor onto which the rotatable plate throws the incoming toner, as taught in Japanese Utility Model Laid-Open Publication No. 130962/1981 (referred to as Prior Art 5 hereinafter). The rotatable plate, therefore, serves to change the direction of transport.
Prior Art 1 is not satisfactory since it does not control the time for returning the collected toner to the developing means or the time for supplementing the fresh toner to the developing means at all. Specifically, Prior Art 1 simply transports the toner sequentially to the developing means at a constant speed when the toner is collected. Since the collected toner is lower in charge than the fresh toner, it is apt to deposit on the background of the image carrier and, when the mixture ratio thereof to the fresh toner is great, it easily contaminates the background of an image on a sheet. When the mixture ratio is not constant, the contamination of the background becomes conspicuous. Particularly, when the apparatus approaches a toner end condition, the fresh toner and the collected toner are easily brought out of balance, aggravating the contamination of the background.
Prior Art 2 supplements the fresh toner and collected toner from the respective storing sections without relating their supplementary amounts to each other. It is likely, therefore, that the supplementary amount of the collected toner exceeds a certain limit to contaminate the background or fly out of the developing means, degrading the image quality and inviting defective cleaning.
Further, Prior Art 3 executes toner supply control only after the image transfer ratio has been lowered and, therefore, cannot immediately cope with the background contamination and other undesirable occurrences. Moreover, Prior Art 3 determines the image transfer ratio in terms of the amount of toner left on the image carrier after image transfer and sensed by an optical sensor. This is undesirable from the stability standpoint since the amount of residual toner on the image carrier greatly depends on the kind of an image (character, solid image, and density). In addition, the problems stemming from the collected toner become more prominent when the image density is lowered than when it is not lowered.
The contamination of the background, scattering of the toner, and other phenomena each occur in a particular manner depending on the change in the potential of the image carrier and the deterioration of the carrier which are ascribable to aging, varying ambient conditions, etc. The conventional image forming apparatuses cannot satisfactorily cope with aging, changes in environment, etc.
Prior Art 4 and Prior Art 5 both return the collected toner to the developing means by mechanical transporting means and therefore, have various problems, as follows. The mechanical transporting means is apt to lower the transport efficiency. Since such transporting means conveys the toner by applying a load on the toner, it is likely to pulverize the toner or to cause the particles of the toner to stick together. The pulverized toner and sticked toner broaden the particle size distribution of the toner to contaminate the background, fly out of the developing means, or cause part of a solid image to be left blank in spots. In addition, the mechanical transport is likely to damage the apparatus due to the fast adhesion of the toner and the stop-up by the toner.